Editing and even viewing (raw) photographs can be a bit taxing on your CPU. If you have an Nvidia GPU, you can supercharge your photo editor by using its OpenCL extensions to offload operations to. Using darktable myself, I had to do a minor bit of research before it worked, as package names have changed in recent Ubuntu.
Assuming you already have nvidia-driver-390 and nvidia-prime installed and switched to use nvidia using
prime-select nvidia (when relevant if you switch between using the built-in Intel GPU and your Nvidia chip for example), there's some packages that need installing before OpenCL works with darktable:
sudo apt install nvidia-headless-390 nvidia-modprobe opencl-headers ocl-icd-libopencl1
Now reboot (or restart X), start darktable and check its settings (little cog icon, 'preferences'). In the 'core options' tab you will now see a checkbox next to 'activate OpenCL support'. Enable and engage :)